Apparatus and method for aircraft cabin noise attenuation via non-obstructive particle damping

ABSTRACT

An apparatus for reducing noise in an aircraft cabin is disclosed. The apparatus comprises a structure portion and filler material. The structure portion further comprises an internal member having at least one cavity disposed therein. Each of the at least one cavity of the structure portion are filled with the filler material.

FIELD OF THE INVENTION

The present invention relates generally to noise reduction apparati andmethods and, more particularly, to an apparatus and method for aircraftcabin noise attenuation via Non-Obstructive Particle Damping.

BACKGROUND OF THE INVENTION

Currently, viscoelastic, or rubber-like, materials are frequently usedin the construction of floor panels and other structural elements inaircraft for both structural vibration and noise energy attenuation, aswell as in other vehicles or structures where so desired. Normally, whenused, these materials come in the form of an adhesive, such as a tape,that is adhered to the surface of the floor panel (or other structuralelement); this adhesive then acts as a structural vibration and noiseenergy absorbing medium. Thus, structural vibration and noise energy isabsorbed via the flexure (i.e., bending) of the viscoelastic materials;this dissipates the mechanical (vibration) energy by converting it intoheat.

Aircraft manufacturers have recently come to utilize honeycombstructures, i.e., structural elements comprising a substantially hollowinterior portion formed by a web of hollow cells or cavities (a fulldescription of honeycomb structural elements is presented below). Due totheir substantially hollow interior, these honeycomb structural elementsare low in both weight and mass, parameters of great importance in thedesign and manufacture of aircraft. However, honeycomb structuralelements are also very stiff. Thus, the degree of any flexure of thesehoneycomb structural elements is small as compared with solid, butheavier, structural elements, such as the floor panels with attenuatingadhesive, as described above. Therefore, the viscoelastic materialsdescribed above are not very effective structural vibration and noiseenergy attenuators when used with regards to honeycomb structuralelements.

Moreover, the effectiveness of structural vibration and noise energyattenuation by viscoelastic materials is highly dependent on both thefrequency of the vibration and the ambient temperature. For example,attenuation by viscoelastic materials does not work well at lowfrequencies. Additionally, viscoelastic materials not only lose theireffectiveness in both low and high temperature environments, but alsodegrade over time, even in ambient conditions.

Thus, there exists a need to develop an adequate structural vibrationand noise energy attenuation apparatus for honeycomb structural elementsthat overcomes the above-stated disadvantages.

SUMMARY OF THE INVENTION

Generally speaking, Non-Obstructive Particle Damping (“NOPD”) is a formof damping in which particles of various materials collide with both oneanother and with the structure in which the particles are located,exchanging momentum and converting vibration energy to heat via frictionbetween the particles. Thus, energy dissipation occurs due to bothfrictional losses (i.e., when the particles either rub against eachother or against the structure) and inelastic particle-to-particlecollisions. In contrast to the viscoelastic materials, which dissipatethe stored elastic energy, NOPD focuses on energy dissipation by acombination of collision, friction and shear damping. NOPD furtherinvolves energy absorption and dissipation through momentum exchangebetween both the moving particles and vibrating walls, as well asfriction, impact restitution and shear deformation.

One advantageous aspect of NOPD is that a high level of damping may beis achieved by actually absorbing the energy of the structure, asopposed to the more traditional methods of damping wherein elasticstrain energy stored in the structure is converted to heat. Thus, with aproper choice of particle size, including density and material, NOPDprovides a very durable and reliable technique of structural vibrationand noise energy attenuation that is essentially independent oftemperature.

Studies have been conducted relating to the general effectiveness ofNOPD in attenuating undesirable structural vibrations and noise energy.As an example, references is made to “Response of Impact Dampens withGranular Materials under Random Excitation” by A. Papalou and S. F.Masri (“Papalou”), the contents of which are hereby incorporated byreference herein in its entirety, which studied the behavior ofparticles in a horizontally vibrating, single-degree-of-freedom (i.e.,one-dimensional motion) system under random excitation. In particular,the Papalou study focused on the influence of mass ratio, particle size,container box dimensions, excitation levels and direction of excitationon various NOPD methods. Design criteria were provided for optimalefficiency based upon reduction in system response.

As a further example, “Structural Damping Enhancement ViaNon-Obstructive Particle Damping Technique,” by Panossian (“Panossian”),studied NOPD in the modal analysis of structures with a frequency rangeof 30 Hz to 5,000 Hz. The method described in Panossian, the contents ofwhich are also hereby incorporated by reference herein in its entirety,consisted of making small cavities at appropriate locations in astructure and partially filling an optimized configuration of thesecavities with particles of different materials and sizes. Significantdecrease in structural vibrations was observed.

To further the strides achieved by the above studies, as well as todevelop a novel and more effective noise reduction apparatus, thepresent invention discloses an apparatus, and a method for constructingand utilizing such an apparatus by a unique application of NOPD. Theapparatus comprises a structure portion and filler material. Thestructure portion includes an internal member defining at least onecavity. Each of the at least one cavities of the internal member of thestructure portion is filled with the filler material of a shape, sizeand density appropriate to achieve the desired damping.

A better understanding of the objects, advantages, features, propertiesand relationships of the present invention will be obtained from thefollowing detailed description and accompanying drawings, which setforth an illustrative embodiment and which are indicative of the variousways in which the principles of the present invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference may be had to oneembodiment, as shown in the following drawings, in which:

FIG. 1 illustrates a perspective view of a section of an apparatus forreducing noise, made in accordance with one advantageous embodiment ofthe present invention;

FIG. 1A illustrates a perspective view of one cavity of the section ofthe apparatus for reducing noise, as illustrated in FIG. 1;

FIGS. 2A-2D illustrate various types and sizes of filler materialdeposited within one cavity of the section of the apparatus for reducingnoise, as illustrated in FIG. 1.

FIG. 3 illustrates various levels of flexural activity, at variousfrequencies, of the apparatus for reducing noise, as illustrated in FIG.1;

FIG. 4 illustrates an amplitude-of-acceleration v. frequency graphcomparing an unfilled structure and various filled noise reductionstructures made in accordance with one advantageous embodiment of thepresent invention; and

FIG. 5 illustrates a model test of noise reduction apparatus madeaccording to one advantageous embodiment of the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY-PREFERRED EMBODIMENTS

The materials an aircraft manufacturer may use during construction aresubject to various design parameters. For example, in addition todesiring that the various structural materials used in the aircraftpossess a high degree of strength, such materials also need a low degreeof flexibility and a relatively low mass. To successfully optimize thesedesign parameters, manufacturers have come to utilize structuralelements commonly referred to as “honeycombs” or “honeycomb structures.”For purposes of the present invention described herein, “honeycombstructures” are structural units (i.e., walls, floors, ceilings, etc) ofan aircraft comprising a substantially hollow middle portion, generallyformed of hexagonal (or other similar shape) rows of hollow cells orcavities, resembling the structure of a honeycomb in a beehive.

One advantage of honeycomb structures is that they can be made of typesof materials both preferred and desired by aircraft manufacturers, whileproviding a sturdy and lightweight alternative to solid structures.However, due to the fact that honeycomb structures comprise asubstantially hollow middle portion, they have a tendency to allow thepassage of structural vibration and noise energy.

Flexural wave velocities, such as those induced by a turbulent boundarylayer located proximate to a structure, such as an aircraft cabin, aregenerally faster than the speed of sound. Unfortunately, honeycombstructures are efficient radiators of these flexural wave velocities atvarious frequencies, including the low end of the spectrum, i.e., thosefrequencies detectable by the human ear. However, it has been found thatNon-Obstructive Particle Damping (“NOPD”) can help reduce this radiatednoise at most of these frequencies. Furthermore, NOPD methods can makeinsulation preferences much easier to achieve and the noise internal tothe cabin much more bearable to passengers. For example, some of theinsulation preferences, such as providing noise amplitudes below 65 dBinside the cabin, are based on the highest noise level that is safe forthe human ear to be exposed to for a period of time.

It should be noted that, although the discussion herein is focused onthe application of the present invention in relation to variousstructures within the interior of an aircraft, it is neverthelesscontemplated that the teachings of the present invention be applicableto other structures wherein a need exists for the attenuation of eitherstructural vibration and/or noise energy. Further, while the teachingsof the invention disclosed herein are focused on the attenuation of bothstructural vibration and noise energy caused primarily by flexural wavevelocities, it is also contemplated that the present invention beequally applicable to such disturbances caused by other means.

Referring now to FIG. 1, there is illustrated a perspective view ofnoise reduction apparatus 10 for attenuating both structural vibrationand noise energy in a variety of situations, such as, for example, anaircraft cabin. Noise reduction apparatus 10 is illustrated ascomprising structure portion 12. Structure portion 12 preferably forms awall, floor panel or other similar element having front member 14,interior member 16 and rear member 18. Preferably, interior member 16comprises a honeycomb structure, as described above. That is, interiormember 16 comprises honeycomb network 20. As illustrated in FIG. 1,honeycomb network 20 is formed by rows of cavities 22. Due to thepresence of cavities 22, honeycomb structure 20 acts to significantlyreduce the total weight of structure portion 12 from that of a solidpanel.

As mentioned above, weight, flexibility and strength are importantfactors to be considered in the manufacture of adequate aircraftmaterials. Thus, it is preferable that front member 14 and rear member18 be comprised of carbon composite, plastic or any other type of lightand relatively strong material, while interior member 16 be comprised ofextremely light and thin sheets of a substantially paper-like and heat-and flame-retardant material, such as that manufactured by DuPont underthe trade name “Nomex™.” Nomex™ is a material uniquely designed for thisspecific purpose, i.e., it is a strong, lightweight carbon compositedesigned of substantially paper-like and heat- and flame-retardantmaterial.

Although illustrated in FIG. 1 as having a hexagonal shape, each cavity22 may be any shape wherein walls 24 extend between and support frontmember 14 and rear member 18. For example, cavity 22 may comprise agenerally circular shape. An example of a hexagonal embodiment of cavity22 is illustrated in the inset, FIG. 1A, of FIG. 1. Additionally, whilenot shown in FIG. 1, it is contemplated that a channel may extendbetween proximate cavities 22, allowing for the transfer of fillermaterial 26 between cavities 22.

To reduce both structural vibration and noise energy in noise reductionapparatus 10, filler material 26 (shown in the inset, FIG. 1A, ofFIG. 1) is deposited within cavity 22 defined within honeycomb network20. Preferably, filler material 26 may comprise separate particles whichmay be metallic and/or non-metallic, or a mixture thereof. For example,metallic particles may be iron, steel, lead, zinc, magnesium, copper,aluminum, tungsten or nickel. Non-metallic particles may be ceramic—suchas zirconium oxide, carbon, and silicon nitride or other silicon-basedhollow materials preferably in the form of micro-balloons—orviscoelastic or rubber-like. Alternatively, metals in the form ofliquids, such as mercury, may be used. A liquid damping material may bepreferred for very low frequencies, while small and light solidparticles are preferred for relatively high frequencies.

Preferably, the particles used for filler material 26 should not belarger than about half the diameter of cavity 22. In practice, thedimensional sizes of the particles should be such that a multiple ofthem should be able to fit within each cavity 22. Although the shape ofeach individual particle of filler material 26 may vary, it is preferredthat the particles be generally spherical hollow micro-balloons.

FIGS. 2A-2D illustrate various types of filler material 26, as depositedwithin each cavity 22. As illustrated by FIG. 2A, filler material 26 maycomprise generally spherical particles. Additionally, these particlesmay also comprise hollow micro-balloons, as described above. Examples ofthese materials are Perlite™ micro-balloons. In FIG. 2B, generallycubic, or crystalline, particles are illustrated as comprising fillermaterial 26. In FIG. 2C, slightly imperfect spherical particles, orgenerally elliptical particles are shown. Finally, in FIG. 2D, fillermaterial 26 is illustrated as comprising irregular-shaped particles.Examples of these particles are sand.

The density of each individual particle of filler material 26 ispreferably related to the mass of each individual particle of fillermaterial 26. Because it is not desired to have a substantial mass incavity 22, lighter density particles, such as, for example, aluminum oraluminum oxide powder, may be used. An additional factor that must beconsidered is viscosity. The more viscous filler material 26, the lowerthe frequency which can be damped. Conversely, the less viscous fillermaterial 26, the higher the frequencies which can be abated. Thus, theviscosity of filler material 26 should be selected depending upon thefrequency range to be attenuated.

In a preferred form, the mass of all filler material 26 which is to bedeposited within cavities 22 of honeycomb network 20 is less than themass of unfilled structure portion 12. It is also preferred to onlypartially fill each cavity 22 with filler material 26, such as, forexample, filling cavity 22 until cavity 22 is 50% to 90% full. Thereason for a partial fill of each cavity 22 with filler material 26 isto reduce the effective noise level as much as the system requirementsallow, while not compromising any weight restrictions or preferences.

Initially, the parts of structure portion 12 are separate parts.Preferably, prior to the deposition of filler material 26 in cavity 22of honeycomb network 20 of interior member 16, rear member 18 is affixedto rear side 28 of interior member 16. Rear member 18 may be affixed torear side 28 through a variety of means and/or methods, such as, forexample by applying a thin coat of strong adhesive on the internalsurface of rear member 18 and attaching, or gluing, rear member 18 torear side 28 of interior member 16. After filling each cavity 22 ofhoneycomb network 20 with filler material 26, to a desired level, frontmember 14 is affixed to front side 30 of interior member 16 (i.e., theside of interior member 16 opposite rear side 28) in, preferably, thesame manner as rear member 18 is affixed to rear side 28 of interiormember 16.

In an effort to arrive at the most optimal solution to overcome thedisadvantages of the prior art, it became necessary to study the dampingeffectiveness of various types of filler material 26 deposited withincavities 22 of honeycomb network 20. These tests were carried out todetermine the effectiveness of NOPD on various honeycomb panels, and todevelop a prediction and design tool that can be used for future NOPDapplication on structures. To this end, a test and analysis program wasinitiated. In this program, Finite Element Model (“FEM”) analyses werecarried out to predict the modal characteristics of honeycomb network 20and to correlate the FEM analyses results with laboratory modal testresults. These tests were then repeated, utilizing various types offiller material 26 and various configurations of honeycomb network 20.During testing, front member 14 was removed, various types of fillermaterial 26 were deposited in cavities 22 of honeycomb network 20, andfront member 14 was re-affixed. The assembled noise reduction apparatus10 was then suspended with rubber bungee cords and structurally excitedby electromechanical shakers. The acceleration and velocity responseamplitudes were measured using a multitude of small accelerometersplaced on the suspended apparatus, and damping values were predictedusing the measured data. The data was then compared with the sametesting procedure using no filler material 26, as well as the procedureusing various types of filler material 26. A Statistical Energy Analysis(“SEA”) was then carried out to predict an acoustic attenuation profilein the frequency range of interest.

In one test, nine forms of apparatus 10 (each having structure portion12 of approximately 2 ft.×2 ft.×0.5 in.) were tested for modalcharacterization using various filler material 26. The FEM analyses andtests indicated numerous vibration modes, the first starting at around63 Hz frequency. Vibration modes illustrate the level of flexuralactivity of a vibrating apparatus 10 at each individual frequency. FIG.3 illustrates the various levels of flexural activity in the vibratingapparatus 10 at various frequencies. As is FIG. 3 illustrates, thevibration modes begin at around 63 Hz.

After performing the FEM analysis, the results were correlated with thetest data to anchor the model such that the model predictions match thetest data more closely. That is, to predict the performance of any fillconfiguration of filler material 26. This FEM analysis was then used forthe prediction of the modal characteristics of any configuration andmaterial content of the apparatus, where each cavity 22 was consideredas an individual solid element.

The FEM analysis predictions initially showed a first bending mode at115 Hz frequency with the uncorrelated model. As illustrated in FIG. 3,and after correlation with test data, the FEM analysis predictions werere-evaluated for the first flexural mode at 63 Hz, as well as atnumerous higher frequency modes. The FEM analysis was then slightlymodified again to correlate better with the test results and to reflectthe mass and damping effects of the various types of filler material 26on the structure portion 12 and correlated with test data. Thiscorrelated FEM was then used for design purposes of future NOPDtreatments of structures.

In the second test, modal and vibration tests were carried out tocharacterize the modal parameters of the nine different structureportion panels. One panel was left unfilled and used as a baseline. Theremaining panels were filled with filler material 26 containing variousparticles and tested under identical suspension and vibration conditionsfor comparison.

Thus, for example, one of the panels was filled with 3M Scotchlite(i.e., 3M Light), having a mass of 0.12 g/cc, and another panel with “3MHeavy,” having a mass of 0.63 g/cc. 3M Light and 3M Heavy particles aregenerally spherical hollow micro-balloons, such as is illustrated inFIG. 2A. The weights of structure portion 12 were measured to calculatethe particle mass of filler material 26 added in each test. The weightof the empty panel was 2187.7 g. The panel filled with 3M Light was2358.2 g, while the panel filled with the 3M Heavy was 2546.9 g.Further, another panel, filled with Aluminum Oxide micro-balloons, was3901.4 g. Thus, the added weight for the 3M Light was only 42.5 g/sq.ft., for the 3M Heavy it was 89 g/sq. ft., and for the AluminumOxide it was 428 g/sq. ft. These added weights represent 7.7%, 16% and78%, respectively, of the total noise reduction apparatus mass.

For purposes of the present invention, “micro-balloons” are, relative tothe size of cavity 22, small particles of filler material 26.Preferably, micro-balloons, as used in the present invention, areair-filled. Due to their high volume and low weight, micro balloons maybe utilized as a preferred filler material for the present invention.Preferably, these micro-balloons have a range of dimensions varyingbetween 300-600 microns in size.

The overlays of the frequency response functions for the empty panel(i.e., the baseline panel), the panel filled with 3M Light and the panelfilled with 3M Heavy particles are illustrated in FIG. 4. FIG. 4illustrates the amplitude, as frequency increases, recorded in panelsfilled with 3M Light and 3M Heavy, as well as a comparison with an empty(i.e., baseline) panel. As illustrated, there are quite a few modes inthe range of 50 Hz to 3200 Hz. The lowest mode was measured at around 63Hz. Damping for this mode was not increased significantly by either ofthe two lighter particles. However, both the 3M Light and 3M Heavyparticles did enhance damping appreciably as frequency increased.Specifically, as the frequency increased from approximately 1000 Hz,both the 3M Light and 3M Heavy panels show a very distinct level ofdamping. As FIG. 3 shows, both 3M Light and 3M Heavy reduced theamplitude of the vibration to around 40 g/lb. A summary of the dampingestimates, as well as the response amplitudes of the structure, aregiven in Table 1. More specifically, Table 1 illustrates the percentageof damping present, at various frequencies, in 3M Light, 3M Heavy, athird material—Aluminum Oxide, and a baseline panel; TABLE 1 PercentagesOf Damping Present At Various Frequencies For Various Filler Material(And A Baseline (i.e., Empty) Panel). Material Frequency PercentageDamping Baseline Panel 152.450 0.246 364.805 0.321 764.809 0.667 779.5450.705 1178.391 0.833 1324.300 0.842 3M Light 147.363 0.387 350.140 0.752727.039 1.327 755.470 1.440 1053.580 1.755 1128.004 2.474 3M Heavy141.863 0.498 340.353 0.941 694.430 1.819 717.478 2.557 1031.280 3.2231103.100 4.821 Aluminum Oxide 93.600 3.500 300.000 4.500 530.000 3.600743.000 6.000

Referring to FIG. 5, the modal tests were conducted by suspending eachnoise reduction apparatus 10 with bungee cords 32 from four points. Fourpoints of contact are illustrated in FIG. 5; however any number ofbungee cords may be used to suspend each noise reduction apparatus 10,provided bungee cords 32 are very flexible and do not effect theresponses of the structure significantly. Accelerometer 34 was placed onnoise reduction apparatus 10 and laser vibro-meter was used to measurethe velocity profile. Both hammer and shaker inputs (not shown) wereused to excite noise reduction apparatus 10 under random and sine dwellexcitations with various amplitudes, to study the nonlinear effects. Themeasurements were then used to identify the mode shapes and frequenciesand to calculate the damping ratios. Fifteen specific modes wereselected and sine dwell excitations were used for modalcharacterization. This data helped the correlation with the FEM analysesand the derivation of the mode shapes. The damping ratios in Table 2show an average increase of 100% for the 3M Light test, over 200% forthe 3M Heavy test and over 500% average increase for the Aluminum Oxidetest. The relative amplitudes illustrated in Table 2 indicate moreamplitude reductions. More specifically, Table 2 illustrates therecorded values of amplitude, at various frequencies, for 3M Light, 3MHeavy, Aluminum Oxide and a baseline panel. As can be seen by Table 2,amplitude for 3M Light Decreased to 5.3 g/lb, while that of 3M Heavydecreased to 2.1 g/lb at an approximate frequency range of 700-750 Hz(as compared with an amplitude of 11.2 g/lb for the baseline panel). Atthis frequency, Aluminum Oxide's amplitude was reduced to 1 g/lb. TABLE2 Amplitude Values Present At Various Frequencies For Various FillerMaterial (And A Baseline (i.e., Empty) Panel). Material FrequencyAmplitude (g/lb) Baseline Panel 152.450 16.15 364.805 20.57 764.80910.89 779.545 11.20 3M Light 147.363 11.05 350.140 7.95 727.039 5.46755.470 5.30 3M Heavy 141.863 8.65 340.353 7.33 694.430 2.72 717.4782.10 Aluminum Oxide 93.600 0.17 300.000 1.45 530.000 1.40 743.000 1.00

Based on the above, it becomes apparent that filling cavities in ahoneycomb structure with micro-balloons provides significant damping ofvibration and resulting noise. More specifically, the lightest type offiller material, 3M Light, provides greater than 50% vibrationattenuation in low frequency modes, as compared with the application ofno filler material. In general, heavier particles provide for a greaterdegree of damping for very low frequency modes. However, as is alwaysthe case when considered in relation to aircraft cabins, the selectionof a type of particle may be subject to weight constraints. Lighter, butmore flexible particles, such as foam particles, could also providesignificant damping when used in a noise reduction apparatus. Moreover,the above-mentioned prediction FEM code is necessary to be able toselect the appropriate particles and fill configuration, and evenpredict the expected responses under excitation. The fundamentalprocedure for the selection and fill configuration of particles for anoise reduction apparatus entails the use analyses and prediction toolsas describes previously. Thus, an optimum configuration and particleselection is possible via the approach described above.

While specific embodiments of the present invention have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure.Accordingly, it will be understood that the particular arrangements andprocedures disclosed are meant to be illustrative only and not limitingas to the scope of the invention, which is to be given the full breadthof the appended claims and any equivalents thereof.

1. An apparatus for reducing noise, comprising: a structure portion, thestructure portion having an internal member, the internal memberdefining at least one cavity; and a filler material, the filler materialbeing disposed within the at least one cavity.
 2. The apparatus of claim1, wherein the internal member comprises at least one sidewall, the atleast one sidewall defining the at least one cavity.
 3. The apparatus ofclaim 1, wherein the internal member defines a plurality is of cavities.4. The apparatus of claim 3, wherein the filler material is disposedwithin at least one of the plurality of cavities.
 5. The apparatus ofclaim 4, wherein the filler material comprises a plurality of particles.6. The apparatus of claim 5, wherein the filler material partially fillseach of the cavities in which it is disposed.
 7. The apparatus of claim1, wherein the filler material comprises a plurality of particles. 8.The apparatus of claim 7, wherein the filler material partially fillsthe at least one cavity.
 9. The apparatus of claim 1, wherein the fillermaterial comprises a silicate.
 10. The apparatus of claim 1, wherein thestructure portion further comprises a front member and a rear member.11. The apparatus of claim 10, wherein the front member defines a top ofthe at least one cavity.
 12. The apparatus of claim 10, wherein the rearmember defines a bottom of the at least one cavity.
 13. The apparatus ofclaim 1, wherein the rear member is affixed to a first side of theinternal member.
 14. The apparatus of claim 13, wherein the front memberis affixed to a second side of the internal member.
 15. The apparatus ofclaim 14, wherein the second side of the internal member is opposite thefirst side of the middle member.
 16. The apparatus of claim 15, whereinthe front member and the rear member combine to bound the at least onecavity defined within the internal member of the structure portion. 17.A method for reducing noise, comprising: providing a structure, thestructure having an internal member; forming at least one cavity withinthe internal member; and; depositing a filler material within the atleast one cavity.
 18. The method of claim 17, further comprisingaffixing a front member and a rear member to the internal member. 19.The method of claim 18, further comprising permitting the randommovement of the filler material within the at least one cavity.
 20. Anaircraft noise reduction apparatus, comprising: an aircraft cabin, theaircraft cabin comprising at least one structure portion, the structureportion comprising a honeycomb network, the honeycomb network having aplurality of cavities; filler material disposed within at least one ofthe plurality of cavities; and wherein the filler material is free tomove within the at least one of the plurality of cavities to absorbstructural vibration and noise energy.
 21. A noise reducing panel,comprising: a first outside portion; a second outside portion; ahoneycomb portion, the honeycomb portion being disposed between thefirst outside portion and the second outside portion, the honeycombportion having a plurality of cavities; a filler material disposedwithin the plurality of cavities of the honeycomb portion; and a meansfor affixing the first outside portion and the second outside portion tothe honeycomb portion; wherein the filler material is free to movewithin at least one of the plurality of cavities to absorb structuralvibration and noise energy.